On the stiffness fairytale...

This is somewhat of an old topic, and I think by now many of us understand that the still ongoing obsession with frame stiffness, in the form of "lateral stiffness", "stiff bottom brackets", etc., etc., is nothing but marketing BS. Yet, just about every single article you see that presents a "review" of some new bicycle, and certainly every single manufacturer blurb about the newest bike from manufacturer X still continues perpetuating this idiocy.

On the other hand, from the physics of bicycles it is entirely clear that frame flex, at the kind of level any halfway realistic, existing road bike will show, has almost exactly zero effect on the efficiency of "power transfer" (whatever that may mean; we note in passing that nobody ever bothers to define this term, and there is certainly not a single test or experiment of any kind that would demonstrate the relevance of this quantity). I have therefore argued for a long time that, if there is any effect of frame stiffness on performance at all, it would have to be sought in the biomechanical efficiency of the rider-bicycle system. However, a priori there is no reason to assume that this system will perform either better or worse using a stiffer frame. As a consequence, it may well be that some riders do better on more flexible frames, and some others may prefer stiffer ones. Bottom line: Nobody knows, and all the rest is nothing but clueless babble.

In recent years, it has become clear that many riders perform better on flexible frames, apparently because it allows them to apply their power more efficiently. Many riders and builders extol the virtues of a “lively” frame made from flexible tubing. When we tested different frame tubing in a double-blind test (Bicycle Quarterly Vol. 6, No. 4), we found that two of three riders preferred the most flexible frame both for constant efforts and for all-out sprints. (The third rider could not tell the – very small – differences between the frames in our test.)

The penultimate paragraph in that piece is also worth quoting in full:

Of course, the real story is more complex. There is more to bicycle performance than overall frame stiffness. Frames can be too flexible for a given rider and application. Some riders may even prefer very stiff frames. However, it is clear that the old mantra of stiffer = more performance is not true for most riders

Amen to that.

Here's my proposal: Can we please lay this BS of the superiority of the stiff frame to rest? Better aerodynamics, yes, lower weight, yes, these things can help increase performance. Frame stiffness? Completely and utterly irrelevant.

Here's my proposal: Can we please lay this BS of the superiority of the stiff frame to rest? Better aerodynamics, yes, lower weight, yes, these things can help increase performance. Frame stiffness? Completely and utterly irrelevant.

Go find a 1995 Litespeed Catalyst and do some sprinting on it, especially if you ride a large size and weigh 200 lbs with 28 in upper legs and report back.

On the flip side, the 58CM SL4 Roubiax lasted 7 months of me putting CGR posts and 27mm Pave tires before i sold it off for being too unforgiving IMO.

So lets assume it is not BS and that the big grey area between too stiff and too flexy is the place riders may find themselves falling comfortably.

Originally Posted by Robt57/Me!

Everything you read that I post is just '1' guy's opinion, try to sort it all out best you can. ;) I will try to add value in my posts, if I miss the mark please let me know using a little decorum.

My own non-empirical experience going from a Ti Litespeed Tuscany to a stiff carbon Cervelo R3 tells me stiffness does matter in performance (improved acceleration and faster average speed). The difference was immediately noticeable.

Go find a 1995 Litespeed Catalyst and do some sprinting on it, especially if you ride a large size and weigh 200 lbs with 28 in upper legs and report back.

That test has been done, with some 1983 steel steed (Pinarello) versus a modern carbon bike (2009 Lapierre). Result, in a nutshell: No difference.

Originally Posted by tvad

My own non-empirical experience going from a Ti Litespeed Tuscany to a stiff carbon Cervelo R3 tells me stiffness does matter in performance (improved acceleration and faster average speed). The difference was immediately noticeable.

The difference was all in your head, at least the difference caused by increased stiffness. Stiffness has no measurable effect on acceleration, and no effect on average speed. Your Cervelo is lighter, hence the better acceleration, and much more aerodynamic, thus higher average speed. Nothing to do with stiffness.

I’m neither a physics or bicycle expert. Nor have I ever stayed at a Holliday Express. So…a question:

When the rider puts down power to the pedals is he not transferring kinetic energy? If that is so then isn’t it accurate that the more the bike frame flexes the greater amount of "loss" of that kinetic energy during that flexion? If that is true than the less the frame flexes the greater the amount of kinetic energy that goes somewhere else? If that is true, where does it go?

Well, yeah, I'm strange this way I know. I'm one of those weird people who thinks that the laws of physics do hold. There are others, like you, who feel their imagination trumps physics. That's what is indeed required to believe in the fairytales of the miraculous benefits of frame stiffness.

Originally Posted by GlobalGuy

When the rider puts down power to the pedals is he not transferring kinetic energy?

No, what he is transferring is force, which gets converted to torque. At the rear wheel the torque gets converted back into a propulsive force. If that propulsive force is not balanced by an equal and opposite resistive force, acceleration results, which increases the kinetic energy of the rider-bicycle system, yes.

Originally Posted by GlobalGuy

If that is so then isn’t it accurate that the more the bike frame flexes the greater amount of "loss" of that kinetic energy during that flexion?

Since this is not so, your antecedent is wrong, too. I'll just leave it at that.

Originally Posted by GlobalGuy

If that is true, where does it go?

If some of the energy input by the rider is used to temporarily flex the frame, once the frame un-flexes the energy is returned. All of it. No energy is lost. None. At. All.

I'll say a good bit of it goes in jolting the bike-rider up and down. The claim BQ is making about "planing" is based on progressively stiffer tubes from a light TT, stiffer DT and stiff CS. The concept is stiff chain stays to efficiently transfer your pedaling energy to the ground and flexier top frame to smooth out the movement oscillation and help the rider pedal stroke sync with the bike.

With bicycles in particular, you need to separate between what's merely true and what's important.

The concept is stiff chain stays to efficiently transfer your pedaling energy to the ground and flexier top frame to smooth out the movement oscillation and help the rider pedal stroke sync with the bike.

Like I said, a fairytale, and nothing but a fairytale.

I will note that it is indeed possible to make the vague concepts you are referring to precise, and test them. It is telling that nobody has ever performed such a test. All the more so given the amount of effort the cycling industry presumably invests into developing ever stiffer frames. Wouldn't you think that, before you invest into a certain characteristic, that you might test whether that characteristic is indeed beneficial? Of course, in this case, the answer is simple, but it's not in the physics but in the marketing, to a scientifically illiterate public.

I will note that it is indeed possible to make the vague concepts you are referring to precise, and test them. It is telling that nobody has ever performed such a test. .

It seems to me that the issue is also heavily weighted towards efficiency of motion and as such can not be holistically tested within the walls of a lab because of the effect of the vary many variables involved with real life cycling and because of the unique reactions each one of us has to them. Creating a model to accurately account for all of these variables would be a Herculean task, possibly not conclusive and with doubtful economic return for the company funding it. Much easier to call a frame "laterally stiff but vertically compliant" and let people gravitate to it.

Regarding BQ, Jan Heine's claims on the flexier frames are based on tests from riding the bikes and cataloging impressions and time trials. Both can be argued against because they seem to be heavily weighted towards perception and circumstances but they seem to work for his style of riding and the equipment he uses.

Nevertheless, different bikes feel different than others and if I like the way the bike rides I really don't care about anyone's tests agreeing or disagreeing with me.

With bicycles in particular, you need to separate between what's merely true and what's important.

If some of the energy input by the rider is used to temporarily flex the frame, once the frame un-flexes the energy is returned. All of it. No energy is lost. None. At. All.

You are ignoring the fact that the energy used to flex the frame is not contributing to propelling the bike forward. It's wasted, whether it is returned or not.

There is no doubt that a frame that directs 100% of the force on the pedals to the rear wheel will be more efficient than one that wastes energy in sideways movement of the BB.
Even if the loss is only a couple of %, that makes a real difference to a pro.

Edit: And this could be tested and proven, although it should be obvious to anyone who thinks about it.
A crank and BB could be attached to a block of steel with a chain to a power meter. Work the pedals with hydraulic or air cylinders that could exert measurable and consistent force.
Recreate the same setup with a flexible BB mount. Measure the difference.

Last edited by Randy99CL; 09-13-2015 at 10:01 PM.

"When you know absolutely nothing, anyone who knows 1% more than nothing sounds like an expert."

How much power would be lost by a rubbing chain on the FD cage because of a flexy bottom bracket. Anything at all measurable? I know it isn't power loss by the frame itself, but still an effect of it if at all significant.

I don't care about the issue, but I don't understand the reference to energy conservation. Energy is conserved, but so what? In thermodynamics you can have two processes that both "conserve energy" but in one case one can't extract "useful work" out of the process and in the other case one can.

Perhaps this conserved energy is telling something interesting about frames, stiffness etc., perhaps not. Quite a few details should be specified before a conclusion is possible.

I can do 80 miles hard on my Addict, and 50 would bring me closer to being tired and slowing in the last 10 that on the 2005 flexy [albeit very comfortable] Roubaix.

Obviously my imagination...

No, you misunderstand. I had said, in my very first post that started this thread, that it is possible that there can be benefits of stiffness to be found in the rider-bicycle system. However, these are subjective in the sense that they can vary from rider to rider. Some riders may indeed benefit, some others won't. There are still many, many cyclists around who love the ride of the typically far more compliant steel bikes, including having no issues with "efficiency".

You are ignoring the fact that the energy used to flex the frame is not contributing to propelling the bike forward. It's wasted, whether it is returned or not.

I am not ignoring anything. The "fact" you mention does not exist.

Originally Posted by Randy99CL

There is no doubt that a frame that directs 100% of the force on the pedals to the rear wheel will be more efficient than one that wastes energy in sideways movement of the BB.

On the contrary, the detailed kinematics of the rider-bicycle system is complex enough so that nobody understands it to the level required to assess the loss, or benefit, of a flexible frame. However, it is easy to see, in principle, how the motion of the frame when returning from a flexed state can indeed contribute to the propulsion of the bicycle. The idea that this sideways motion of the bottom bracket necessarily wastes energy is naïve, given that there is no energy dissipated in the frame itself.

Originally Posted by Randy99CL

Edit: And this could be tested and proven, although it should be obvious to anyone who thinks about it.

This only seems obvious to someone who does not understand the physics. What you find obvious is simply wrong.

Originally Posted by Randy99CL

A crank and BB could be attached to a block of steel with a chain to a power meter. Work the pedals with hydraulic or air cylinders that could exert measurable and consistent force.
Recreate the same setup with a flexible BB mount. Measure the difference.

If you perform this experiment, and do it right, you'll find exactly zero difference. Anybody with a minimal understanding of the requisite physics will understand that, which is why your "test" is pointless. But by all means, feel free to perform that test.

Well, then why don't you explain how you know it's those elusive benefits of stiffness of that Cervelo of yours that is responsible for your presumably better performance, rather than the clear advantages of lower weight and better aerodynamics. Alternatively, let us know what percentage of your increased performance is caused by the three variables, respectively.

For a typical recreational rider plugging along and averaging 14-17 MPH, it's not just stiffness that's a fairytale, but also weight and aero. Ignorant people who keep upgrading to newer bikes which they think will make them faster are fools believing the marketing hype who are being parted with their money. It's very unfortunate that so many bike manufacturers and bike shops rely on perpetuating this myth for much of their business. The only people who should be worrying about faster bikes are racers competing for money who have trained as hard as they can and need every advantage. These people are a tiny minority of the bike buying population.

I was once one of those dummies who believed a lighter and stiffer carbon bike could make me faster. Now I know the truth: that a custom frame that is built to fit me, for my weight, and with desired ride quality will motivate me to ride more frequently and for longer distance. This is what will make you faster. The most efficient vehicle in the world will still be slower without a powerful motor.

It's very curious that grand tour average speeds haven't increased significantly over the years, despite bikes being lighter, stiffer and more aero, tires having less rolling resistance, ceramic bearings, improved training and nutrition, etc.

Then there's the fact that no one bike brand consistently dominates races. It's constantly a different brand of bike winning a stage or a race. And riders on the same team riding the same brand of bike have times varying by many minutes or hours. That is all the proof anyone needs that the rider makes a far bigger difference than the bike.

There are a couple of companion BQ blog posts that came late last year that help explain long-debated issue of frame flexibility, power transfered, performance, and importantly overall rider perception. They can be found here (November 2014), and here (December 2014).

As one well schooled in physics I wrestled with this in my own mind for years. In the second of those links (December 2014) the blog authors note a series of finite element analysis simulations which found that the work done to flex the frame during pedaling is returned to the drivetrain on the return (spring rebound). If the finite element analysis simulations run for BQ's work are representative and valid, then they resolve a big part of the question. Yes the frame flexes, so how much power is lost between the pedals and wheels as a result of that frame flex? The answer is very, very little. The remaining source of power lost would be the hysterical loss between material deformation and rebound. For the rigid materials typically used in bike frames, and the small elastic deflections that would be rational, that loss is probably negligible. So, from a purely physical look it appears that frame flexibility is, in fact, not a source of power losses.

The great debate over power loss and frame flexibility seems to revolve mostly around differences in perceptions between different riders. Part of that is possibly due to a difference in perceived effort between frames of different stiffness. That perception is grounded in the resistance the rider experiences when he applies force to the pedals (it's in their legs, not their head). It extends then to apparent acceleration. If the BQ test results are representative, a rider should be able to develop more power on the more flexible frame. I suspect, however, this has a limitation, and that for any given rider there is an optimum flexibility that facilitates the optimal development of sustained power output for that rider. Regardless, the impact of frame flexibility on a rider's ability to develop power complicates the issue, as it focuses on the rider's ability rather than the original question of applied power lost to work being done to flex the frame back and forth.

The BQ work makes a key point, which I believe is understated. At the end of the day it's about the total performance of bike + rider that matters most. That includes a riders ability to develop power as well as the bikes response to that input. Both of those will probably vary between different riders as well as riders with different objectives at varied times.

"When man invented the bicycle he reached the peak of his attainments."
-Elizabeth Howard West